A gas turbine is a continuous-flow machine, in which a gas under pressure expands. It consists of a turbine or expander, an upstream compressor and an interposed combustion chamber. The principle of operation is based on the cycle process (Joule process): The latter compresses air by means of the blades of one or several compressor stages, then mixes this in the combustion chamber with a gaseous or liquid fuel, ignites it and burns it.
A hot gas (mixture of combustion gas and air) thus forms, which expands in the following turbine part, wherein thermal energy is converted to mechanical energy and initially drives the compressor. The remaining portion is, in the turboshaft engine, used for driving a generator, a propeller or other rotating consumers. The expanded hot gas is guided into an exhaust-gas section which, for this purpose, is surrounded by a heat-resistant shroud, the latter forming the boundary for the hot-gas duct of the exhaust-gas section. At the same time, the inner shroud of the exhaust-gas section separates the latter from the internal parts of the stator arranged around the axis.
Due to constructional constraints, a gas turbine exhaust-gas section cannot be adequately sealed off from the surroundings. Owing to thermal expansion during operation at the last turbine stage on the hot-gas side of the rotor, an axial gap is present between the platforms of the rotor blades and the inner shroud of the exhaust-gas section, the gap connecting the exhaust-gas section to a wheel-side space, that is to say to a space adjacent to the blade wheel. Since the exhaust-gas section can have a positive pressure in relation to the surroundings in the case of unfavorable operating states or hardware configurations, leakage of hot exhaust gases into the surroundings or into the gas turbine enclosure cannot be ruled out. This gives rise to an increased temperature load on the components of the gas turbine and emissions of harmful exhaust gases. The latter presents a safety risk.
The abovementioned unfavorable operating states are small hot-gas mass flows, that is to say low power of the gas turbine, which, due to increasing flexibilization, are becoming ever more important. Furthermore, relatively high exhaust-gas pressures can occur as a result of aerodynamic changes at the last turbine stage or increased exhaust-gas pressure losses in the boiler.
In some gas turbine types, a negative pressure which arises in regions of the exhaust-gas section is used in order to draw in air from the surroundings via an air inlet in the wheel-side space and an air duct connected thereto and to use this air for the purpose of cooling. In particular here, directly after the last rotor blade row of the turbine part of the gas turbine in the flow direction of the hot gas, pressures occur during normal operation which lie below the ambient pressure of the air in the surroundings of the gas turbine. Corresponding openings can thus be made here which draw in air from the surroundings via cooling ducts so that this air can be used for cooling this region, in particular also the wheel-side space located nearby which communicates with the bearing space of the rear turbine bearing. This principle is, for example, described in GB 1270959 or WO 2012/141858 A1.
As described above, however, the problem arises that as the machine power decreases, the exhaust-gas pressure rises and the negative pressure therefore drops. If the exhaust-gas pressure exceeds the ambient pressure, a flow reversal occurs in the air duct and consequently significant leakage of exhaust gas occurs via this opening. This is to be avoided at all costs since, due to the air ducts provided, very large exhaust-gas leakage quantities can occur. There are currently no measures which could prevent exhaust-gas leakage here. This effectively limits the flexibility of the gas turbine, in particular in the range of small loads, since excessive exhaust-gas leakage must be reliably avoided.